The invention relates to reactors and methods for cleaning organic materials from semiconductor wafer surfaces and/or for "ashing" photoresist from semiconductor wafer surfaces.
In semiconductor device manufacturing processes, photoresist is used as a mask for etching and ion implantation. The photoresist is removed after the etching or implantation process is complete. Various techniques for removing the photoresist are well known. Such removal of photoresist is referred to as "ashing".
One known ashing technique, referred to as a thermal ozone process, is widely used. This technique eliminates the potential for ion damage and charge up, which is a consequence of the most popular plasma processes, but requires high reaction temperatures of more than approximately 300.degree. Centigrade. This technique requires use of an expensive (roughly $60,000.00) ozone generator. The high reaction temperatures cause reliability problems, one of which is occurrence of miniature explosions of hardened crust on top of the photoresist, referred to as "popping", causing defects in the integrated circuit.
Another type of ashing process is referred to as UV ozone processing. UV ozone ashing techniques can be performed with lower reaction temperatures, and therefore avoid the above "popping" problems. One UV ozone technique, which has been used in equipment marketed by Hitachi for several years, uses mercury resonance lamps having a wavelengths of mostly 254 nm (nanometers) and some of 185 nm. Since the 185 nm intensity of the lamp is too low to achieve a reasonable ozone content, this system has the shortcomings that an expensive ozone generator is required, and the mercury resonance lamp provides inadequate light intensity. Consequently, the reaction rate involving production of an oxygen radical to decompose organic contaminants or photoresist is too low.
Other UV ozone processes and systems which utilize xenon excimer lamps are marketed by Usiho of Japan and Tokyo Cathode Laboratory of Japan. The xenon excimer lamp has a peak output at 172 nm. An oxygen molecule has a high absorption coefficient at this wavelength. Therefore, it can effectively generate ozone and does not require an expensive ozone generator. However, the ozone produced has a low absorption coefficient at that wavelength, resulting in a low reaction rate. The xenon excimer lamp has instantaneous turn-on/turn-off characteristics, unlike the mercury resonance lamp which requires several seconds to stabilize the light intensity after turn-on. This is a significant amount of time in ashing of semiconductor wafers, wherein a wafer is loaded into and unloaded from the ashing reactor approximately every other minute. Photons of the 172 nanometer wavelength quite effectively modify/alter the organic photoresist material composition of the form C.sub.l H.sub.m O.sub.n. Unfortunately, the reaction rate is too low if ordinary low power xenon excimer lamps are used. Equipment including higher power xenon excimer lamps which would provide a higher reaction rate is not presently commercially available. Furthermore, xenon excimer lamps have the problem that they produce solarization or colorization of the Suprasil quartz windows which separate the lamp chamber and reaction chamber of an ashing device, resulting in degradation of the transmitted light intensity.
The reaction for the above prior art system using a xenon excimer lamp is given by: EQU O.sub.2 +h.nu.(172 nm).fwdarw.O(.sup.3 P)+O(.sup.1 D), (Eq.1)
wherein O(.sup.3 P) and O(.sup.1 D) represent oxygen atoms of excited electronic states that react with oxygen gas to produce ozone, as indicated in Eq. 2: EQU O+O.sub.2 +M.fwdarw.O.sub.3 +M, (Eq. 2)
where M represents a foreign gas. EQU O.sub.3 +h.nu.(172 nm).fwdarw.O.sub.2 (.sup.1 .DELTA.)+O(.sup.1 D),(Eq. 3)
wherein O.sub.2 (.sup.1 .DELTA.) represents an oxygen molecule of excited electronic state, and O(.sup.1 D) represents an oxygen radical. Ozone has a low absorption coefficient at the 172 nm wavelength emitted by a xenon lamp. Because of this low absorption rate and low light intensity of the low power excimer lamp or rapid light intensity degradation due to solarization of the high power excimer lamp, it is not very effective for ashing or cleaning a wafer.
Thus, there remains an unmet need for an improved ashing and cleaning system and method which overcomes the problems of the prior art.